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Interpretive Summary: Medusahead is a problematic annual grass, and managing medusahead requires an integrated strategy that combines herbicides, prescribed burns, and revegetation with desired species. Herbicides used to control medusahead can potentially interfere with establishment of seeded species. We tested how imazapic rate and timing impacted medusahead, seeded species, and other nontarget vegetation on burned and unburned rangeland. Medusahead cover was highest in plots that received no imazapic and lowest in plots that received the highest rates. Seeded species and other nontarget species varied in their response to imazapic, and their response depended on site and burning treatment.

Technical Abstract: Medusahead is an introduced, winter-annual grass covering millions of acres of the semiarid West. It forms exclusive stands and has a dense thatch cover that resists the establishment of desirable species. Prescribed fire can remove medusahead litter and improve establishment of desirable plants. With or without fire, medusahead control is key to establishing desirable vegetation that will, then, hopefully resist further invasion. Imazapic is an effective herbicide for pre- and post-emergent control of medusahead, but more information is needed on its effects on desirable species. The objectives of this study were to test how imazapic application rate and timing impacted medusahead, seeded desirable species, and other nontarget vegetation on burned and unburned rangeland in SE Oregon. A 2-hectare portion of an existing medusahead infestation was burned at two different sites in June 2003. Following the burn, imazapic was applied monthly at rates of 0, 35, 70, 105, 140, 175, and 210 g ai/ha between July and October of 2003 in a randomized strip-plot design replicated three times. In November 2003, monocultures of seven desirable species were drill-seeded across the imazapic treated areas. Data on cover and density of medusahead, seeded species, and other nontarget species were collected in summer of 2004 and 2005. Medusahead cover was highest in the plots that received no imazapic and lowest in plots that received the highest herbicide application rates (140 g ai/ha – 210 g ai/ha). Medusahead cover also was lower in burned plots, though this pattern was less visible during the second year after burning. The effect of imazapic on nontarget vegetation was less clear. Seeded species established in the study plots, but the effect of herbicide rate on them showed few consistent patterns; some of the seeded species showed little response to herbicide, while others appeared to establish best at different herbicide rates depending on site and whether the plots were burned or unburned. We also found no clear patterns relating imazapic rate or application month to cover of perennial or annual grasses or forbs; these results were also dependent on site and whether the plots were burned or not.